The picturesque lightshow that, once the sun has set, takes place on the hour, every hour, when the Eiffel Tower is lit up for five minutes by thousands of coruscating bulbs, stops with a final spasm at 1 a.m. It was unlit when I reached it at 3 a.m. on a damp Monday morning. The surrounding streets were deserted.

When I set off across the Champs de Mars in order to stand beneath the Tower, my footsteps disturbed several rats that had been eating from the ruined litterbins full of tourists’ droppings. The rats loped across the path in front of me and disappeared into the dirty pools of darkness beneath the nearby trees — it reminded me of the repulsive landscape described by Robert Browning in his dream-like quest-poem, "Childe Roland to the Dark Tower Came," where the grass grows "as scant as hair in leprosy," and rats shriek like babies.

I felt frightened. If I’d seen anyone else standing or walking in the precincts of the Tower, I’d have panicked and run. There was no one. Perhaps that was more ominous. In Umberto Eco’s novel Foucault’s Pendulum, a taxi driver admits to the narrator that at night he always feels compelled to accelerate past the Eiffel Tower because it scares him. Why? "Parce que … parce que ça fait peur, c’est tout." I too felt that fear — and couldn’t remain an instant longer. So I rapidly retraced my steps to the rue de l’Université, spooked by the thought of the Tower rearing up implacably behind me. I felt as if a layer of skin had been scraped from my back beneath my neat, black rucksack, and thick clothes.

Turning into the avenue de La Bourdonnais I slid into a dark dreamscape: a handgun lay on the doorstep of a building to my left; solid, geometric, shocking. It must have been dropped on the stone step by someone running up the avenue, or tossed from a passing car. I tried to remember the emergency number as I pictured the man who had left it… then imagined him returning for it and finding me. Perhaps it was one of the young men who, an hour or so earlier, had almost run me down at a junction, hip-hop pumping from their car as they shouted incomprehensible abuse.

As I peered more closely at the gun I realised with a creeping sense of the ridiculous that it looked, well, plastic. Peering closer, I saw it didn’t have a trigger, and must have been bought in one of those shops retailing things, stuff, crap for €1.50 a pop. Behind me, sagging in the rain, a cardboard box filled with broken toys and kids’ clothes had been abandoned on a bench.

Although steeped in noir clichés, my conspiratorial delusion wasn’t implausible. In April, a 43-year-old homeless Bulgarian man had found a cache of weapons discarded near the Place Stalingrad. Sifting through rubbish bins in search of food, he came across one that contained a 6.35-calibre automatic pistol, four grenades, sixteen rounds of ammunition, and a shell designed for the sort of armoured vehicle used by the French army. Beside a dump truck nearby was a wooden box full of grenades.

The police ascertained soon enough that these arms belonged to a local "collector" — the term sounded euphemistic to me — who had recently died. But it wasn’t entirely clear why the man’s relatives, however desperate to have them off their hands, had dumped the weapons in public bins in a densely populated part of Paris. "This was a completely inconsiderate act," a police representative commented humourlessly, "because the weapons were still operational and could have fallen into the wrong hands."

In Paris, in the fourth year of an economic recession used by both centre-left and centre-right administrations to punish the poorest sections of the population, homeless Bulgarian men who rummage through bins in the city centre are not often thought to possess the right hands. Theirs are dirty, grasping, thieving hands. According to the authorities and the right-wing press, the Roma who have arrived in France in increasing numbers over the last five years, migrating from Bulgaria and Romania, are beggars, pimps, prostitutes, and petty criminals.

As many as 20,000 Romanichels, or Gitans, as they’re known in France, subsist in abject conditions in makeshift camps on the suburban margins of the capital and other cities. There they are often the victims of vigilante attacks by local residents, like the one that ended the life of a sixteen-year old Roma known as Darius near the A1 motorway in June 2014. They are also the target of violent expulsions by the police. The forcible eviction of these camps and the repatriation of their inhabitants commenced under Nicolas Sarkozy, prompting one EU Commissioner to compare it to the persecution of Jews during the Occupation. It has continued, even been extended, under François Hollande. Amnesty International claims that more than 10,000 Roma were evicted from temporary camps in the first half of 2013.

The pavements of central Paris are currently home to several hundred Roma. In the course of the night I spent traversing the city on foot I felt I glimpsed most of them, their features distinctive among the scattered forms of the white and black homeless people that populate the streets. Some slept alone. At 1 a.m., on the Place des Vosges, where Cardinal Richelieu lived in the early seventeenth century, four or five of them were huddled under the arcades in frayed sleeping bags. At 4 a.m., on Boulevard Haussmann, single Roma men slept in the entrances of department stores, their bodies heaped beneath blankets, clothes and rags, their grimy faces often encircled by scarves. Some were stretched out beside the road — in spite of the rain — on mattresses and even battered wooden doors that had been removed from their frames, They looked as if they had been washed up on life rafts.

Other Roma slept together. Couples had crammed themselves into phone booths dotted along the main avenues and boulevards. In one of them, the woman sat huddled at the feet of the man, who stood to attention and stared blankly at me as I passed. In another, the woman’s body was contorted into a V-shape. Her head, at waist height, was wedged in one corner; her naked feet were wedged in the opposite corner. There was something so intimate about the sight of the woman’s calloused soles, pressed up against the Perspex surface of the booth, that I felt as if simply in glancing at their flattened form I had intruded on her.

Most unnerving of all were the Roma families, who face a struggle to find accommodation in emergency shelters originally designed for single men (many of these hostels have in any case been forced to close down because of government cuts). Sometimes three or four children slept in the entrance of a shop beside their mother and father. These doorways were so heaped with small sleeping bodies, thick clothes and faded quilts that, perversely, they communicated a sense of cosiness. The miserable expressions of their parents as they watched over the children dispelled this illusion.

So did the presence of others condemned to remain on the street in the night — the people against whom these parents guarded. A hundred feet from where the Roma families squatted, I came across six or seven prostitutes. It was 4.30 a.m., and they seemed mystified by the grim determination with which I marched through the rain. Five minutes later, escaping from the downpour in a doorway, I watched two men of North African origin fighting one another. They threw hard, slippery punches, and one of them knocked the other to the ground, sending him skidding along the slick surface of the pavement. A bottle shattered. "Ta mère! Ta mère!" — "Motherfucker!" — the man who was still standing screamed.

Roma were the most striking presence on the street that night, but innumerable other destitute or desperate people became visible in the city after dark. At about 2 a.m., close to the Musée d’Orsay, I leaned over the parapet above the Seine because of a commotion. Three young men were scrawling something on the embankment while a security guard looked on in bemusement. One of the men hurled himself at the stone wall and smashed his forehead against it repeatedly. When he finally rolled back onto the ground, rocking on his spine with a strangely acrobatic elegance, then staggered to his feet again, the guard reluctantly lifted a mobile phone to his ear.

I had begun my noctambulisme shortly before 9 p.m. on Sunday, walking northeast from the Place de la République to the périphérique. After a coffee I climbed the hill to Belleville. At the entrance to the Couronnes metro station a black man in his fifties and a white man in his twenties sifted through a tangled stack of broken furniture, like nineteenth-century ragpickers, and poked at the rotten vegetables spilling out of crates that had been dumped there. On the rue des Couronnes, an elderly, bearded man in a foetal position slept heavily on the pavement, piss leaking from his trousers and forming a runnel as it snaked down the steeply sloping street.

On the terrace above the Parc de Belleville — where I watched the shivering lights of the Eiffel Tower signal 10 p.m. in the far distance — there were forty or fifty people standing around some trestle tables waiting for something to happen. Africans, Arabs, Eastern Europeans, Roma; and, looking cheerfully incongruous, two dilapidated white men on crutches. Almost everyone clutched shopping trolleys or carried empty bags and holdalls. The rain was only falling lightly, and there was a companionable, if not festive, atmosphere. Some young people were drinking and smoking and listening to a tinny radio. Kids were running around squealing. Everyone was talking, in a riot of different languages.

Then a van arrived and several men unloaded its contents onto the tables, lit by a string of bare bulbs. Suddenly the terrace resembled a market: precarious piles of fresh fruit and vegetables; boxes filled with biscuits and pasta; neat stacks of school exercise books, kids’ rucksacks; nappies, tampons. A food bank. Everyone crowded round the trestle tables, but they behaved with generosity, a lack of urgency, in waiting to receive things. Emblazoned on the organisers’ t-shirts was a slogan, "La Main de l’autre."

The bobos — the bohemian-bourgeois class who cultivate what Eric Hazan calls a "superficial non-conformism" — have in recent years moved into Belleville in droves, driving up rents and pushing out immigrants, marginals, and the poor. But there was no sign of them this Sunday night. Once the bars and cafés have closed, Paris stubbornly resists gentrification. In the night the Roma and other poor and homeless people — those whom the streets have claimed — reclaim the streets. Only the Eiffel Tower, it seems, emblem of the city of light, encompasses a completely lifeless space.